There are a number of reasons not to attend such a ceremony (which is invalid to begin with). I am separating out different reasons, each of which is enough to make it wrong to attend. Since I am writing this edit on Purim, I will point out that even if it is kosher, it is like the Jews attending the celebration of Achashveros.
As explained in Orthodox Visits to Other Denomination Sanctuaries Rav Moshe is cited as having stated that one cannot go to a wedding in a Reform Temple (especially if it takes place in the sanctuary as opposed to a social hall). This applies even if this is supposed to be between two Jews. Additionally, if the person officiating is a reform or Conservative rabbi it would probably not be valid either (even if not in a Reform venue as sapecified in the question).
The second part (Reform conversion) can be addressed by pointing out that the convert is not Jewish and this is like going to any other intermarriage of a Jew and a nonJew. An example of this question is Intermarriage Attendance as well as Invited to Engagement of Girl intending to marry out: Attend/Not-attend?.
These point out that even an "engagement party" should not be attended because it appears as if one is accepting such a "marriage". As shown in A People of Destiny
Against this background of destiny and identity many halachic social
constraints on Jewish-gentile relations can be understood and, in
paramount, the egregiousness of intermarriage stands out.
Intermarriage Rachaman litslan destroys Jewish identity and prevents
the rendezvous with Jewish destiny. Accordingly, the Rav zt"l was
absolutely adamant regarding the ban on attending an intermarriage.
With this same compelling line of reasoning in mind, the Rav was also
equally adamant that subsequent to the wedding intermarried "couples"
must not be included in family gatherings or invited to family
semachos, and the like. Inviting the couple as such eo ipso
acknowledges and accepts their illicit marriage. Under no
circumstances may this acceptance be forthcoming. Let us not delude
ourselves into thinking that we would be simply maintaining relations
for purposes of kiruv. To the contrary, we are being m'sa'yai'a yedei
ovrei aveira, strengthening the hands of those living in sin and
creating a chilul Hashem.